Remove legacy JS processing

What does this MR do and why?

• Removes processing for legacy JavaScript in the content/assets/javascripts directory. We no longer have legacy code that needs versioning or special filtering by Nanoc.
• Updates dev docs on JavaScript usage.
• Updates dev doc on analytics (!4317 (merged)).

Closes #439 (closed) 🎉

Screenshots, screen recordings, or links to review app

No visual changes.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Verify that you can compile the site without errors
3. Verify that scripts in content/assets/javascripts still get copied over by viewing a page with a Mermaid diagram, like this: https://439-remove-legacy-js.docs.gitlab-review.app/ee/user/markdown.html (we intentionally do not bundle Mermaid through Rollup because it's super slow)

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.

Closes #439 (closed)